1. Technical Field
The present disclosure generally relates to battery power sources, in particular to batteries comprising circuitry for charge and discharge control, and to methods of operating batteries.
2. Description of the Related Art
Rechargeable batteries are used as a power source for various devices, from small devices having a relatively low power consumption such as mobile phones, digital cameras and media players to devices having a high power consumption and/or using a voltage of up to several hundred volts such as electric vehicles and hybrid vehicles, for example electric and hybrid cars. Besides lead acid batteries, nickel cadmium batteries, and nickel metal hydride batteries, lithium ion batteries have recently gained popularity, due to their high energy to weight ratio, absence of memory effect and slow loss of charge when not in use.
In applications wherein a voltage that is greater than the voltage of a single cell (approximately 2 volts for a lead acid battery cell, 1.2 volts for nickel cadmium and nickel metal hydride battery cells, and approximately 3.6 volts for lithium ion battery cells), a plurality of cells are electrically connected in series. For achieving higher operating voltages of about 200 to about 300 volts or more, as may be used in electric or hybrid car applications, a large number of cells are electrically connected in series.
Battery cells can be damaged when they are used outside an allowable voltage range, both when they are charged to a voltage that is greater than a maximum allowed voltage, and when they are deeply discharged, such that the voltage between the poles of the cell is smaller than a particular minimum voltage. In particular, lithium ion cells can be susceptible to damage outside of an allowable voltage range.
In a battery comprising a plurality of cells, the individual cells can have a different capacity, for example due to production tolerances, uneven temperature distribution in the battery, and differences in the aging characteristics of individual cells. Moreover, the individual cells in a battery can have a different state of charge, due to fluctuations that may occur in the charge or discharge process of the battery.
If there is a degraded cell in the battery having a diminished capacity, or a cell having a higher state of charge than the other cells, there is a danger that once this cell has reached its full charge during a charging process of the battery, it will be subject to overcharging until the other cells of the battery reach their full charge. The result can be temperature and pressure build up and possible damage of the cell. During discharging of the battery, a cell having a smaller capacity than the other cells, or a lower state of charge, will have the greatest depth of discharge and will tend to fail before the other cells. It is even possible for the voltage on the cells having the lowest state of charge and/or the smallest capacity to be reversed as they become fully discharged before the rest of the cells, which may lead to an early failure of these cells.
For overcoming these problems, it has been proposed to balance the charge of the individual cells in a battery, using specific charge balancing circuits connected to the cells of a battery and/or incorporated into the battery.
A further issue that may occur in the operation of batteries, in particular in the operation of lithium ion batteries, is avoiding an overcharge or a deep discharge of the entire battery. For this purpose, and also for controlling charge balancing processes, batteries comprising a plurality of individual cells, in particular lithium ion batteries are provided with a battery control circuit. A battery control circuit according to the state of the art will be described with reference to FIG. 1.
FIG. 1 shows a schematic circuit diagram of a battery 100 comprising a stack 103 of cells 104, 105, 106, 107. In addition to cells 104-107, further cells can be provided, as schematically indicated by dots between cell 106 and cell 107. In FIG. 1, cells 104 to 107 are denoted by common circuit symbols, wherein a longer line denotes the positive pole of the cells 104 to 107, and a shorter line denotes the negative pole of the cells 104 to 107.
The negative pole of the stack 103 is connected to a negative terminal 102 of the battery 100. The positive pole of the stack 103 is connected to a drain 114 of a discharge field effect transistor 110 (abbreviated as “DFET” in the following). A source 113 of the DFET 110 is connected to a source 112 of a charge field effect transistor 109 (abbreviated as “CFET” in the following), whose drain 111 is connected to a positive terminal 101 of the battery 100. Hence, the DFET 110 and the CFET 109 are electrically connected in series between the positive pole of the stack 103 and the positive terminal 101.
The CFET 109 and the DFET 110 can be power metal oxide semiconductor transistors. In some batteries according to the state of the art, the CFET 109 and the DFET 110 can be n-channel transistors, wherein the sources 112, 113 and drains 111, 114 are n-doped, and the body of CFET 109 and DFET 110 is p-doped. While the body of CFET 109 and DFET 110 is internally connected to the source of the respective transistor, the pn-transition between the body and the drain has a rectifying property, as schematically illustrated by diode symbols 117, 118. Note, that diodes 117, 118 are not circuit elements that are provided in addition to CFET 109 and DFET 110, but are part of an equivalent circuit illustrating features of CFET 109 and DFET 110.
CFET 109 and DFET 110 can be controlled by applying a voltage to gate 115 of CFET 109 and gate 116 of DFET 110.
In the normal operation, both CFET 109 and DFET 110 are switched on. Thus, the battery 100 can be charged and discharged through CFET 109 and DFET 110. For preventing an overcharge or a deep discharge of the cells 104-105 in stack 103, CFET 109 and/or DFET 110 can be switched off. Thus, current flow in and out of the stack 103 can be prevented.
For controlling the operation of CFET 109 and DFET 110, a control unit 108 can be connected to gates 115, 116 of CFET 109 and DFET 110. Additionally, the control unit 108 can comprise circuitry for performing charge balancing between cells 104 to 107, as schematically illustrated by connections between each of the cells 104-107 and the control unit 108.
If the CFET 109 and the DFET 110 are switched off, the voltage of the entire stack 103 is applied at the DFET 110 in the reverse direction of diode 118, and a charging voltage applied between the positive 101 and negative 102 terminal of the battery 100 is applied in the reverse direction of the diode 117 of CFET 109. For preventing an undesirable charging or discharging of the battery 100 and/or a destruction of CFET 109 and DFET 110, the breakdown voltage of CFET 109 and DFET 110 has to be greater than the voltage of the entire stack of cells 103 and the voltage that is applied when the battery 100 is charged.
A problem of the battery 100 according to the state of the art is, that if the stack 103 comprises a large number of cells 104-107, the voltage of the stack 103 and the voltage that is applied when the battery 100 is charged can be relatively high (several hundred volts or more), which can make it difficult, if not impossible, to provide CFET 109 and DFET 110 with a sufficiently high breakthrough voltage.
Therefore, it has been proposed to omit the function of a separate charge and discharge control for a high voltage battery such as a battery for an electric or hybrid vehicle.
However, in this case, the battery has to be dimensioned in way that the battery is operated in a range between about 40 percent and about 80 percent of its total capacity to ensure the functionality over the lifetime of the battery. Moreover, the lifetime of the battery may be adversely affected, since the battery has to be replaced after the capacity of the battery has decreased to 80 percent of the starting capacity.